Injection molding is well known as a method of obtaining a molded product by supplying molten resin injected from an injector to a cavity formed in a mold, and then curing the molten resin by cooling.
In injection molding, resin is melted in an injector, and obtained molten resin is injected from the injector and flowed in a hot runner. The molten resin is further led to a product part forming a part of cavity, via a spool, a gate, and the like formed in a mold. Just before (an upstream side of) a cavity, a nozzle may be disposed as described in Patent Document 1, for example, and the molten resin may be led out from a nozzle and supplied to a cavity.
A temperature of a hot runner is kept at 200-220° C., for example, and a temperature of a mold is substantially at a room temperature. Therefore, the molten resin injected into a cavity is deformed (namely, molded) along the shape of the cavity, cured while decreasing a temperature by heat being removed, and formed as a molded product.
In such injection molding, it is attempted to decrease the amount of resin to lower the product cost, or to produce a thin-wall product with a small thickness dimension (wall thickness) for the purpose of obtaining a lightweight molded product to decrease CO2 emissions. However, in this case, if the injection conditions are the same as those for molding a thick-wall product, a flowing distance of molten resin may be shorten.
When such a situation occurs, for example, molten resin does not reach an end of the product part. In other words, a filling defect occurs and a corresponding portion is lost, or so-called deformation occurs and a defective molded product is produced.
For avoiding such a problem, it is considered to increase a molten resin injection pressure. In this case, a pressing force to molten resin is increased, and a flowing distance of molten resin is expected to increase. However, when a molten resin injection pressure is increased, a burr is likely to occur on a parting surface, especially near a gate. Thus, it is also considerable to increase a mold clamping pressure to narrow a gap causing a burr as much as possible. However, for obtaining a high mold clamping pressure, a large size or high power displacement mechanism is necessary for mold clamping/opening by displacing a movable mold. Thus, an injection molding apparatus is increased in size and weight. Further, such a displacement mechanism is generally expensive, and increases a capital investment.
From the above viewpoint, Patent Document 2 has proposed a method of molding a resin molded product having a thin-wall portion at a low equipment cost. The method comprises forming first and second resin paths in a mold, opening a valve provided in the second resin path after molten resin that is led to a product part from the first resin path passes through a portion forming a thin-wall portion, and supplying the molten resin from the second resin path to an unfilled portion of the product part.
In the related art described in the Patent Document 2, for storing a valve opening timing, or a timing of supplying molten resin from the second resin path, in a control unit, it is necessary to previously determine a relationship between an elapsed time after start of molten resin injection and a molten resin reaching position in a product part by repeating tests. This is troublesome, and takes a long time for performing the tests.
As described in the paragraph [0027] of the Patent Document 2, it is also considerable to provide a detection unit in a product part, and to detect that molten resin passes through a predetermined position by using the detection unit. However, in this case, a molten resin injection pressure must be changed depending on the position of the detection unit.
Thus, it is considered to increase a temperature of molten resin and decrease a viscosity of molten resin by setting a high temperature for melting resin in an injector, and to lead the molten resin into a cavity after maintaining the high temperature/low viscosity state in a hot runner. However, in this case, according to the inventor's intensive studies, a molded product with an insufficient strength may be often produced due to a change in physical properties of the molten resin.
Moreover, the injection molding apparatus disclosed in the Patent Document 2 is of a so-called multipoint gate type, in which a plurality of gates is present for leading molten resign to a product part. Thus, a weld line is formed, degrading the external appearance quality of molded product. In addition, a plurality of valve gates must be provided, increasing mold costs.